An automatic clutch between the motor and drive train of a motor vehicle with a gearbox, which is arranged on the output side of the clutch, and can be shifted by the driver, is known from WO 98/13620. A positioning unit used for engaging and disengaging the clutch is controlled by sensors taking preset values into account, whereby the sensors have signal generators for the respectively engaged rotational speed and/or the respectively engaged gear as well as signal generators for rotational speed detection arranged on the input side and output side of the clutch, and whereby the engaging and disengaging of the clutch are controlled as a function of the rotational speed differences between the clutch input and output. In the event that the signals of the signal generators are no longer available for the rotational speed on the output side, replacement signals will be calculated using other values. In particular, it is proposed that the clutch output rotational speed is determined on the basis of a gearbox output rotational speed that has remained the same since the last phase of the previous engaged state of the clutch and taking into account the respectively engaged rotational speed.
It is further known that knowing the gearbox output rotational speed is especially important in the control of automated gearboxes. Therefore, an ascertained rotational speed signal of a rotational speed sensor at the gearbox output shaft is usually compared, with regard to its plausibility, with the wheel rotational speed signals of sensors at the rear wheels of the motor vehicle, whereby these wheel rotational speed signals are also available for controlling an anti-lock braking system or an anti-slip regulating system (ABS/ASR system). By means of such plausibility checking of the determined rotational speed values of the transmission output shaft, the following malfunctions should especially be prevented:
a) the measured gearbox output rotational speed is equal to zero and therefore indicates that the motor vehicle is standing still, while it is actually moving. In this operating situation, automatic shifting of the first gear as a start-up gear can result in the destruction of the clutch by an inadmissibly high rotational speed of the gearbox input shaft.
b) the measured gearbox output rotational speed is greater than zero, and therefore indicates a vehicle rotational speed, while the vehicle is actually standing still. This circumstance can lead to an erroneous detection of the vehicle motion, and thus to automatic engagement of the clutch, so that the vehicle is unintentionally started because the automated clutch is engaged according to known control methods during motion, even without the deflection of the accelerator pedal in order to use the braking effect of the prime mover.
Rendering the ascertained rotational speed signals usually takes place such that an arithmetic mean value of sensor-detected rotational speeds of both rear wheels of the motor vehicle is formed. The rear differential ratio is taken into account, and the calculated input rotational speed of the rear axle gearbox thus ascertained is compared to the sensor-detected gearbox input rotational speed. The arithmetic mean of the rotational speed signals of both rotational speed sensors at the rear wheels is used for averaging the rotational speed differences between both rear wheels that are to be balanced by the differential gear while cornering.
With this intrinsically advantageous plausibility check of the rotational speed it cannot, however, be ruled out that the signal transmission of at least one rotational speed sensor at the rear wheels may fail due to damage to the rotational speed sensor itself, or due to failures in the signaling lines and/or their electric contacts. As a result of such false report, which is detrimental to further operation of the motor vehicle, for example start-up shifting of the gearbox during motion is prevented subject to the current operating state, or the gearbox is shifted into neutral while the vehicle is standing still, as a result of which the vehicle can no longer be started up. Both known control reactions limit and/or prevent operation of the vehicle.